The present invention relates to superconductors, particularly ceramic superconductors and to processes for making same.
The term "ceramic superconductor" has been coined recently to describe new superconducting materials which offer the promise of superconductivity at higher temperatures than previously thought attainable. In particular, many of the ceramic superconducting materials announced by various researchers throughout the world during the early months of 1987 superconduct at temperatures equal to or greater than that of boiling liquid nitrogen at atmospheric pressure. As used in this disclosure, the term "ceramic superconductor" means a material which exhibits superconductivity at some temperature and which incorporates a plurality of metals together with a chalcogen. By far, the majority of ceramic superconductors incorporate oxygen in conjunction with three or more metals, typically including copper, a Group II metal selected from the group consisting of calcium, strontium, barium and mixtures thereof, and a transition metal selected from the group consisting of Sc, Y, La, the other lanthanides which include Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and mixtures of these elements. Among the most promising ceramic superconductors are the "1, 2, 3" superconductors incorporating the transition metal, Group II metal and copper in molar ratios of 1:2:3 and also including oxygen. Other known ceramic superconductors are listed hereinbelow.
Superconductors such as ceramic superconductors most typically have been made heretofore by processes involving consolidation. In one such process, mixed metallic-oxide particles including the various metallic constituents of the final superconductor in conjunction with oxygen are consolidated by subjecting the particles to a final pressing step followed by a final sintering step. Ordinarily, the oxygen content of the material is adjusted during the final sintering step, as by adjusting the temperature and/or oxygen content of the surrounding atmosphere so as to change the equilibrium oxygen content of the material. The mixed metallic oxide particles employed in the final pressing and sintering steps typically are formed in preliminary steps which provide for mixing of the various metals included in the starting materials. Thus, the process may start with substantially pure compounds of the individual metals to be incorporated in the final superconductor, in finely divided, particulate form. These pure compounds may be intimately mixed and sintered in one or more preliminary steps. The resulting intermediate material may then be pulverized, remixed and resintered. This process may be repeated so as to progressively intermix the various metallic elements by interdiffusion prior to the final consolidation steps.
Other processes employed heretofore involve consolidation of a solid superconductor from a vapor or liquid phase containing the constituents of the superconductor rather than from particles. Thus, processes such as chemical vapor deposition, sputtering, Pfann zone melting, plasma spraying, hot pressing and isostatic hot pressing have been employed to form superconductors.
The manifest commercial potential of superconductors useful at relatively high temperatures has spurred what may fairly be characterized as an unprecedented research effort involving many of the physicists and material scientists in the world. However, apart from the fundamental discovery of to the existence of ceramic superconductors, much of the work done to date has focused on characterization of known ceramic superconducting materials or minor modifications of known materials.